The turbocharger aims at achieving high output of an engine using energy of exhaust gas discharged from the engine, and is widely adopted in various conventional engines.
In the engine equipped with a turbocharger, since a boost pressure reaches a limit pressure when an engine speed is increased to a certain extent, it is necessary to perform a boost pressure control for preventing the boost pressure from exceeding the limit pressure.
For instance, the turbocharged engine described in Patent Document 1 includes: a bypass passage that bypasses a turbine of the turbocharger; and a waste gate valve provided in the bypass passage. When the engine speed becomes equal to or higher than an intercept speed (a rotational speed when the boost pressure reaches the limit pressure in full load condition), the waste gate valve is opened and part of the exhaust gas flows in the bypass passage. Thus, an amount of exhaust gas flowing into the turbine is reduced and an increase in the boost pressure is suppressed.
The engine of Patent Document 1 further includes: an EGR passage mutually connecting an exhaust passage and an intake passage of the engine; and an EGR valve provided in the EGR passage. In a high speed area equal to or higher than the intercept speed, while the waste gate valve is opened, the EGR valve is closed. Thereby, a flow of the exhaust gas passing through the EGR passage is cut off, and an operation of returning the exhaust gas from the exhaust passage to the intake passage (Exhaust Gas Recirculation) is stopped.
Even by, as Patent Document 1, opening the waste gate valve and closing the EGR valve at the intercept speed and resultantly suppressing a flow rate of the exhaust gas flowing into the turbine, when the engine speed is further increased from the status and the flow rate of the exhaust gas increases, pressure of the exhaust gas (the exhaust pressure) in the upstream of the turbine gradually increases. On the other hand, since the boost pressure, in other words, a pressure of the intake passage in the downstream of the compressor, is maintained constant, an increment of the exhaust pressure with respect to the boost pressure becomes large as the engine speed is increased. Thereby, a pumping loss is increased, which leads to degradation in fuel economy performance.